1. Field of Invention
This invention is related to absolute position encoders. In particular, this invention is directed to systems and methods for spatially synchronizing coarse and fine resolution measurements in absolute position encoders.
2. Description of Related Art
Absolute position encoders, such as those disclosed in U.S. patent application Ser. No. 09/213,268 and U.S. Pat. No. 5,841,274, each incorporated herein by reference in its entirety, generally have two members movable relative to each other.
In the absolute position encoder disclosed in the 268 application, each transducer includes at least one transmitter winding and at least one receiver winding. In these transducers, the transmitter windings are inductively coupled to the receiver windings by the same plurality of coupling loops. In particular, in the 268 application, the first coupling loop portions 206 are arranged at a different wavelength than the second loop portions 208. Accordingly, depending on which transmitter winding is driven, the scale will couple to the receiver windings 224 or 226, respectively, at a different effective wavelength. Thus, depending on which transducer is being driven, a different fine wavelength measurement can be taken. By combining the phase position information obtained by driving the two transducers in sequence, a coarse wavelength position can be determined, as described in the 268 application.
The absolute position encoder disclosed in the 274 patent includes two or more position transducers. In the 274 patent, each position transducer includes at least one transmitter winding for generating a magnetic field and at least one receiver winding for receiving an associated magnetic field. Each position transducer also includes a plurality of flux modulators which modulate the magnetic field generated by the transmitter winding. Each receiver winding has a plurality of loops arranged along a measurement axis. The receiver windings of two of the position transducers define two wavelengths that are similar to each other, but not equal. Thus, a phase relationship between the outputs of the two position transducers has a coarse wavelength much longer than the wavelength of either of the two individual position transducers. Therefore, the absolute position between the two members can be determined over the range of the coarse wavelength.
Alternatively, the absolute position encoder can include only one fine wavelength position transducer. Such an absolute position encoder also includes one or more position transducers having coarser (longer) wavelength(s). The absolute position between the two members is determined by first identifying a coarse absolute position along the measuring axis using the coarse wavelength position transducer, and then refining this coarse position using information from the fine wavelength position transducer. A coarse wavelength transducer can be based, for example, on an analog scale, using a sine function or a triangular function, or a parallel or serial digital code track scale.
In some absolute position transducers, the coarse and fine scales need to be spatially synchronized or calibrated. This has been accomplished, for example, by placing the read head at one location somewhere on the scale and activating a command for spatially synchronizing the fine and coarse scales. The command is activated for instance by pressing a zero button for a long time (one second or more). When this is done, a value is stored in an electrically programmable memory, such as an EEPROM, that corresponds to the offset in the coarse wavelength position measurement relative to the fine wavelength position measurement at that particular location. This single offset is then used for the entirety of the scale.